Guide

Static electricity and humidity

Why do I get static shocks in winter?

Because dry air stops surfaces conducting. Above roughly 40% relative humidity, a molecule-thin film of water covers most surfaces and lets electrical charge leak harmlessly away. Below that the film stops forming, charge accumulates instead, and it discharges through you.

The mechanism

Static charge is generated constantly — walking across a carpet, sliding out of a car seat, pulling off a jumper. Two materials touch, electrons transfer, and they separate carrying opposite charges. That happens in July as much as in January.

What changes is whether the charge stays.

Above roughly 40% relative humidity, most surfaces carry an adsorbed water film only a few molecules thick. It is invisible and it is enough: it makes the surface very slightly conductive, so charge drains away as fast as it is generated. Nothing accumulates.

Below about 40%, the film stops forming. Charge has nowhere to go, so it builds — until you reach for a doorknob and it finds a path.

The air itself is not the conductor. Humid air is still an excellent insulator; the discharge is across surfaces, not through the room.

Why the season is so sharply defined

Static often starts and stops within the same week or two each year, which feels odd for something driven by a gradual variable.

The reason is the same arithmetic behind every dry-air complaint. Cold outdoor air carries very little water however high its percentage reads. Bring it inside and warm it and no water is added, but the amount the air could hold rises steeply — so the relative humidity collapses.

Outdoor air brought inside and warmed to 21°C. Nothing added, nothing removed.
Outside	Water carried	Indoors reads	Static?
10°C at 70%	6.6 g/m³	36%	Occasional
5°C at 80%	5.4 g/m³	30%	Noticeable
0°C at 80%	3.9 g/m³	21%	Routine
−10°C at 90%	2.1 g/m³	12%	Constant

The threshold sits right where autumn turns — so a cold snap crosses it in a couple of days, and the whole household notices at once. The forecast that morning will still say 80% or 90% humidity, which is why the connection is not obvious.

This is the clearest everyday demonstration of why the absolute figure matters and the percentage alone misleads.

What actually helps

Raise the indoor humidity. A humidifier is the direct fix, and getting above 40% is usually enough to stop it entirely.
Change what is on the floor. Wool and synthetic carpet generate far more charge than hard flooring.
Wear cotton. Polyester, nylon and fleece sit far from skin and wool on the triboelectric series, so they separate much more charge.
Discharge deliberately. Hold a key and touch that to the door first — the spark jumps from the key, not your fingertip. Same nerve endings, no pain.
Hold the car door frame as you get out. The charge equalises gradually through a large contact area instead of arcing across a small one.

Anti-static sprays work by leaving a hygroscopic residue that attracts exactly the water film the dry air is preventing — the same mechanism, applied locally.

Why it matters beyond the nuisance

For most people this is only irritating. In some settings it is not:

Electronics. A discharge you can barely feel is thousands of volts and can damage semiconductors, which is why assembly work uses grounded mats and wrist straps.
Fuel handling. A spark near vapour is a genuine ignition risk, which is why static discharge procedures exist at fuel depots.
Printing, textiles and powders. Sheets cling, fibres misbehave, and dust clings to everything.

All of them are managed by raising humidity or by providing a deliberate path to ground — the same two options as your hallway.

What this site can tell you

The outdoor water content, which is what determines how dry your indoor air becomes when that air comes inside and warms. That makes it a reasonable early warning for a static-prone spell.

It also estimates the likely range for a given room from those outdoor figures and how the room is used, or reads a HomeKit sensor if you have one. Neither the phone nor a weather service senses your house directly — the estimate is a typical figure for that kind of room, and a measurement needs an instrument in the room.

Common questions

What humidity stops static electricity?

Above roughly 40% relative humidity static largely stops being a nuisance, and above 50% it is rare. Below 30% it becomes routine. The threshold is gradual rather than sharp, and it is about the surface humidity rather than the room average.

Why does static only happen in winter?

Because cold outdoor air carries very little water, and warming it indoors adds none — so the indoor relative humidity collapses. It is not the cold or the heating directly, it is the arithmetic of warming already-dry air.

Does humidity conduct electricity?

Not the air itself — humid air is still an excellent insulator. What conducts is the thin water film that forms on surfaces when the ambient humidity is high enough. The charge leaks across surfaces, not through the air.

How do I stop getting shocked by the car door?

Hold the metal door frame as you get out, so you and the car equalise gradually through a large contact area rather than through a fingertip. Touching a key to the door first also works — the spark happens at the key rather than your skin.

Why do some fabrics cause more static?

Because materials differ in how readily they give up or accept electrons — the triboelectric series. Synthetics like polyester and nylon sit far apart on it from wool and skin, so rubbing them together separates a lot of charge. Cotton is much closer to neutral.

Sources

ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy — Source of the 30–50% comfort range

Published 12 August 2026